This invention concerns a device for hooping an object by means of a heat-weldable plastic strip strapped around it.
A hoop-casing device of the aforementioned type is known for instance from U.S. Pat. No. 3,269,300 and comprises a means for tensioning the strip and a means for friction-welding two mutually overlapping strip portions of the tensioned strip between two welding jaws. Each of these means is assigned a motor, a switch for controlling this motor via a control circuit, and a cam for actuating the switch. The two cams can be rotated jointly about a common rotation axis. One of the welding jaws can be displaced in a direction essentially orthogonal to the rotation axis while being adjustable by means of a cam gear. The cam gear comprises a cam disk that can be rotated about the rotation axis along with one of the cams, and a telescopic tappet that is functionally arranged between the cam disk and the displaceable welding jaw and is telescopically retractable or extensible in a direction essentially othogonal to the rotation axis and is spring-loaded loaded in its direction of extension.
A drawback of this known hoop-casting device is that the force that presses the welding jaws against the plastic strips depends on the latter""s thickness. If the pressing force is too great the motors are demanded too much power output, the rotation speed of the motors drops, and the hoop-casing device no longer operates properly. Nevertheless, in this known hoop-casing device there is provided no adaptation capability that would allow to use plastic strips of various thickness values.
Accordingly, the object of the invention is to provide a hoop-casing device of the relevant generic type that does not have the aforementioned drawback and hence, that will allow for the use of plastic strips of various thickness values.
In this embodiment of the hoop-casing device according to the invention, the maximum force that urges the welding jaws towards each other is determined by the rotational position of the cam disk relative to at least one of the cams, all the more as this maximum force appears at an extremal rotational or angular position of the cam disk that results from at least one of the cams having a restricted rotation capability in the course of its rotation about the rotation axis relative to the device. The invention allows this maximum force to be adjusted by means of the adjustment of the rotational position of the cam disk relative to said cam and thus, to be selected depending on the thickness of the plastic strips, which allows to use the hoop-casing device with plastic strips of various thickness values. For example, plastic strips of 0.4 mm to 1.05 mm thickness can be used, with the force that presses the welding jaws onto the plastic strips being adjustable so as to keep almost the same value, so that the hoop-casing device always operates properly independent from the thickness of the plastic strips.
Advantageous embodiments of the hoop-casing device according to the invention are defined in the dependent claims.
More particularly, one of the cams can be assigned a stop projection to limit its rotation about the rotation axis in cooperation with a stop fixedly arranged at the device, which will determine the extremal rotational or angular position of the cam disk that, for its part, will determine the maximum force that urges the welding jaws towards each other.
The followings combination may be deemed especially advantageous: The cams each are arranged at a respective assigned shaft part. The rotation axis is shared between both shaft parts. The two shaft parts can be separated from each other between the two cams or connected to each other rigidly with regard to rotation in respect of each other by means of a mutually meshing gear. This gear is embodied as a longitudinal gear having generatrices oriented parallel to the rotation axis. In the vicinity of a respective end portion thereof, one of the two shaft parts is embodied as a trunnion and provided with an external toothing and the other one is embodied as a sleeve part provided with an internal toothing. In the vicinity of their respective trunnion-shaped or sleeve-shaped end portion, the two shaft parts can be coaxially inserted into, displaced relative to, and separated from, each other. The cam disk embodied as an eccentric cylinder is arranged at a shaft part that is rotatable relative to, and fixed in axial direction relative to, the device, whereas the other shaft part is arranged at the device so as to be rotatable as well as displaceable in axial direction. The shaft part that is displaceable in axial direction is supported by means of a spring element at a housing portion that is fixedly arranged at the device, arranged to protrude from the housing portion, and stressed by the spring element towards the shaft part that is fixed in axial direction. Thus, the shaft part that is displaceable in axial direction can be pulled out manually at the housing portionxe2x80x94with the help of a knob provided for this purposexe2x80x94and then angularly displaced, upon its release the pulled-out shaft part will tend to mesh again with the other shaft part: the relative rotational position of the shaft parts thus arrived at will determine the relative rotational position of the stop projection and the cam disk and hence, the extremal rotational or angular position of the cam disk and the maximum force that urges the welding jaws towards each other, in this manner this urging force can be adjusted to fit the thickness of the plastic strips.
In combination With the preceding, there may also be deemed especially advantageous: The one longitudinal toothing is provided with a bridgework region in which a plurality of teeth appear to be fused together from tip to tip when viewed in cross-section, this one longitudinal toothing thus being provided with a filling. The other longitudinal toothing is provided with a teeth gap region in which a plurality of teeth have been omitted from root to root when viewed in cross-sectional, this other longitudinal toothing thus being provided with a recess. The bridgework region extends across a smaller number of teeth and thus, with respect to the rotation axis, across a circular arc of smaller extent than the teeth gap region. In this manner the two shaft parts may only be connected i.e. mesh one into the other over a predetermined sensible region, whereas unreasonable and hazardous operating conditions are avoided.